The restriction you mention only applies if he has been granted subsidiary protection after 17 March 2016. In this case your parents will not be allowed to apply for Family reunion until 16 March 2018.
The rule is: He must be 17 when the German embassy in Beirut and the Ausländerbehörde decide on the application for a visa for your parents. The date of the application is not relevant. There are possibilities to enforce a fast treatment by court applications in case you have the impression that your case is not handled fast on purpose.
But in his case the time frame between 16 March 2018 and his birthday in summer 2018 may be too short. I am not aware of any other ways. (In this context you may hear about the famous "fristwahrende Anzeige" (§ 29 AufenthG); however this does not apply to your case, only to parents (in Germany) who want to bring their children to Germany.)
But you should also keep the following in mind:
- Only your parents are allowed to apply for a visa for family reunion. Other sisters/brothers can only apply once your parents are in Germany with a residence permit.
- The visa/residence permit of your parents expires on the date your brother turns 18; they will have no automatic right to stay on further, they will need a different ground to apply for residence permit. (They may be granted a residence permit due to their status as parents only if they are too old and in need of care by your brother). So it is questionable whether they can stay on.
He will receive ALG II (He is in school? in this case BAföG may apply and what he receives there will be deducted from AGL II)
To my knowledge you can be insured as a family as long as he is under 18 and if you live together.